1. Field of the Invention
The present invention relates to a fuel injection type internal combustion engine equipped with a plurality of intake valves.
2. Description of the Related Art
The fuel consumption of an engine is reduced as the air-fuel mixture fed into the engine cylinders becomes lean. Consequently, in order to reduce the fuel consumption, it is preferable to make the air-fuel mixture as lean as possible. However, where a lean air-fuel mixture is used, a problem occurs in that good ignition cannot be obtained. In addition, even if the lean air-fuel mixture is ignited, since the operating velocity of the flame is low, another problem occurs in that a good combustion cannot be obtained.
In order to eliminate such problems, an engine is known in which the intake passage extends tangentially to the inner circumferential wall of the combustion chamber. A fuel injector is arranged in the intake passage, and the fuel injection from the fuel injector is stopped at a time slightly before the intake valve closes (Japanese Unexamined Patent Publication No. 56(1981)-148636).
In this engine, when the engine is operating under a partial load, only air is fed into the combustion chamber of the engine at the former half of the intake stroke, fuel injected from the fuel injector is fed into the combustion chamber at the latter half of the intake stroke. Consequently, the layer of rich air-fuel mixture is formed in the uppermost interior of the combustion chamber, and the layer of lean air-fuel mixture is formed in the lower interior of the combustion chamber. Thus, the air-fuel mixture in the combustion is stratified. As a result, since the rich air-fuel mixture is collected around the spark plug, the air-fuel mixture can be easily ignited. In addition, since a swirl motion is created in the combustion chamber by an air stream flowing into the combustion chamber from the intake passage, the flame of the air-fuel mixture ignited by the spark plug can be rapidly propagated.
In an engine in which the air-fuel mixture in the combustion chamber is stratified, where the mean value of the air-fuel ratio of the entire air-fuel mixtures stratified in the combustion chamber is maintained as a constant, the higher the degree of stratification, the better the combustion. For example, where the mean value of the air-fuel ratio of the air-fuel mixtures overall is 25:1, a good combustion can be obtained when the air-fuel ratio of the rich air-fuel mixture is 15:1 and the air-fuel ratio of the lean air-fuel mixture is 35:1, as compared with the case where the air-fuel ratio of the rich air-fuel mixture is 20:1 and the air-fuel ratio of the lean air-fuel mixture is 30:1. That is, it is possible to ignite the rich air-fuel mixture even if the air-fuel ratio thereof is 20:1. However, in this case, the flame of the burning rich air-fuel mixture is weak, and thus, since it takes a long time to burn the lean air-fuel mixture, it is difficult to obtain a good combustion. Contrary to this, where the air-fuel ratio of the rich air-fuel mixture is 15:1, the flame of the burning rich air-fuel mixture becomes strong. Consequently, in this case, even if the lean air-fuel mixture is much leaner, the time period necessary to burn the lean air-fuel mixture becomes short, and thus it is possible to obtain a good combustion. Therefore, as mentioned above, in order to obtain a good combustion, it is necessary to enhance the degree of stratification.
However, in the above-mentioned engine disclosed in Japanese Unexamined patent publication No. 56-148636, since the fuel injected from the fuel injector flows into the combustion chamber at a high speed together with air, the fuel is easily spread widely in the combustion chamber. Therefore, in this engine, in spite of the stratification of the air-fuel mixture in the combustion chamber, a problem occurs in that the degree of stratification is not sufficiently enhanced.
In order to eliminate such a problem, the present applicant has already proposed an engine equipped with two intake valves (Japanese patent application No. 59(1984)-69176). In this engine, the first intake passage connected to the combustion chamber via the first intake valve is formed in a helical shape, and an air control valve, which is open when the engine is operating under a heavy load, is arranged in the second intake passage connected to the combustion chamber via the second intake valve. The fuel injector is arranged in the second intake passage downstream of the air control valve.
In addition, the present applicant has also proposed an engine equipped with a third intake valve in order to improve the volumetric efficiency when the engine is operating under a heavy load (Japanese patent application No. 59(1984)-103124).
In these engines proposed by the present applicant, when the engine is operating under a partial load, the air control valve arranged in the second intake passage is closed. Consequently, when the second intake valve opens, the level of vacuum in the second intake passage downstream of the air control valve immediately increases to a level near the level of vacuum in the combustion chamber. As a result, since the pressure difference between vacuum in the combustion chamber and vacuum in the second intake passage downstream of the air control valve is small, fuel injected into the second intake passage downstream of the air control valve flows into the combustion chamber at a very low speed. Consequently, in these engines, the fuel is less easily spread in the combustion chamber, and thus it is possible to enhance the degree of stratification.
However, in these engines, if the operating time of the second intake valve is set so that the second intake valve is open at the latter half of the intake stroke, in order to enhance the degree of stratification, since the flowing speed of the fuel into the combustion chamber from the second intake passage is very low, the entire amount of fuel injected into the second intake passage cannot be fed into the combustion chamber, and the fuel which has not been fed into the combustion chamber is fed into the combustion chamber as soon as the second intake valve opens in the next intake stroke. However, as mentioned above, if the fuel remaining in the second intake passage is fed into the combustion chamber when the opening operation of the second intake valve is started, the fuel is positioned at the central portion of the combustion chamber. This results in a problem in that a good stratification of the air-fuel mixture in the combustion chamber cannot be obtained.
Contrary to this, if the opening time of the second intake valve is advanced, in order to feed the entire amount of fuel in the second intake passage into the combustion chamber, the fuel in the second intake passage is fed into the combustion chamber from the early part of the intake stroke, and thus it is difficult to obtain a good stratification of the air-fuel mixture in the combustion chamber. That is, where the engine is constructed so as to stratify the air-fuel mixture in the combustion chamber by closing the air control valve arranged in the second intake passage, it is impossible to obtain a good stratification independently of the opening time of the second intake valve.